There has been a need to replace metallic stents with those formed from biodegradable materials. However, any such replacement needs to maintain the advantages of metallic stents, that is, the strength and resistance against compressive forces that tend to reclose the lumen opened by the stent.
It is known to provide biodegradable thermoplastic stents that are deployed by heating the plastic until it softens, and then expanding the softened stent until it achieves a desired diameter, such as that of the body lumen in which it is inserted. For example, U.S. Pat. No. 5,670,161 discusses a tubular stent so processed, as well as (in column 2) prior art tubular stents in Beck et al, U.S. Pat. No. 5,147,385. Particularly as to the latter, it explains that the '385 stent is heated above its melting temperature (the polymer "enters a liquid phase in the deployment! temperature that Beck discloses"), and hence "improved strength characteristics using the stent described by Beck is limited". The solution of the '161 patent is to use a copolymer of the Beck homopolymer, the copolymer having melting temperatures that greatly exceed the deployment heating temperature so that there is no melting of the copolymer stent.
Thus, the trend as shown by the '161 patent is to avoid melting a tubular thermoplastic stent when it is deployed by heating and expanding it, as this weakens the strength properties of the stent. That is, the entire plastic tube of the '385 patent melts, thus losing its integrity and its inherent strength. However, the "solution" of requiring only the use of a copolymer is one that is undesirable due to the limited ability to resist compressive forces in any new expanded form. There has been a need, therefore, to provide a process of using a thermoplastic stent by heating and expanding, that is not limited just to single materials but which retains the strength properties of, e.g., copolymers.